Currently, Long Term Evolution (LTE) systems do not support voice calls, so Circuit Switched Fallback (CSFB) specified by 3GPP standards TS23.272 is executed for these calls. CS fallback function and SMS (short message system) message delivery via the CS core network is realized by reusing Gs interface mechanisms as defined in 3GPP TS 29.018 on the interface between the MME (Mobility Management Entity) in the EPS (Evolved Packet System) and the VLR (Visitor Location Register). This interface is called the SGs interface.
For example, the network is enhanced to page the mobile over the LTE radio, and when a mobile camped on LTE radio receives a voice call page request over the LTE system, the mobile device falls back to the CS radio to receive voice call. In connection with CSFB, any active data session is suspended or handed over to 2G/3G PS (packet switched) access. For Mobile Originating (MO) calls, the mobile devices or user equipment (UE) camped on the LTE radio initiate fallback on their own to originate a voice call in the CS domain. Mobiles camped on LTE radio can send and receive SMS messages without falling back. The basic idea is to force the UE to fallback to CS radio for voice calls, which is not VoLTE (Voice Over LTE). In order to execute the CSFB, a new interface, called SGs, is required between MME (Mobility Management Entity) and MSC/VLR (Mobile Switching Center/Visitor Location Register) to allow the MSC to know when a UE is EPC (Evolved Packet Core) attached. The MSC/VLR registers with the HLR (Home Location Register) as normal, so that Mobile Terminating (MT) voice calls and SMS messages are routed there. In these cases, MO/MT SMS messages are delivered over LTE access and EPC via the MSC. For MO calls, the UE falls back to CS radio before originating the call so no core network changes are required. For MT calls, the call is delivered to the MSC as normal, and the MSC pages the UE via the new SGs interface between the MME and the MSC. The UE falls back to CS radio and issues either a Location Update or Page Response. The MSC completes the call. If the UE falls back to a MSC different than the one that issued the SGs page, complex “roaming retry” procedures are required to complete the call. That said, this process requires new interfaces and modification of existing nodes in each of the LTE and CS systems. The new SGs interface is required between the MSC in the CS core network and the MME in the EPC. The SGs enables the MSC to learn of EPC attachment, MO/MT SMS during EPC attachment, and MSC to page the subscriber over EPC and LTE access for voice calls. As for the modified nodes, the MSC requires SGs interface and associated state machine. The MME requires an SGs interface and modification of the associated state machine and requires S3 interface to SGSN if ISR (Idle mode Signaling Reduction) is enabled. The MSC/HLR/GMSC all require changes in order to support Roaming Retry, which is made more likely by CSFB.